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The development of underwater electroacoustic transducers expanded rapidly during the twentieth century, and continues to be a growing field of knowledge, with many significant applications, one that combines mechanics, electricity, magnetism, solid state physics and acoustics. In the most general sense, a transducer is a process or a device that converts energy from one form to another. Thus an electroacoustic transducer converts electrical energy to acoustical energy or vice versa. Such processes and devices are very common. For example, a thunderstorm is a naturally occurring process in which electrical energy, made visible by the lightning flash, is partially converted to the sound of thunder.

This chapter will describe the six major electroacoustic transduction mechanisms in a unified way using one-dimensional models to derive pairs of linear equations specific to each mechanism as discussed in general in Section 1.3. Important characteristics of the transducer types will be summarized and compared to show why piezoelectric and magnetostrictive transducers are best suited for most applications in water.

Active sonar and acoustic communication systems rely on electroacoustic transducers which “project” sound that is subsequently detected by hydrophones through a direct path or reflection from a target. Our focus in this chapter is on the projector which is significantly larger and more complex than the hydrophone because of the need to generate high acoustic intensity.

All the applications of underwater projectors described at the beginning of Chapter 3 also require the use of hydrophones. In most active sonar systems the same transducers serve as both projectors and hydrophones, but there are good reasons in some cases to use separate hydrophones for reception, (e.g., hydrophones in towed line arrays can be well removed from the ship’s self-noise). In addition passive search and surveillance sonar, as well as passive ranging sonar, use only hydrophones. Passive sonobuoys and various noise monitoring functions also require only hydrophones.

Naval applications are the main motivation for the development of large, innovative sonar systems. Therefore, the development of large acoustic arrays is closely related to new ship construction, and especially to new submarines since they depend so strongly on acoustics [1, 2]. The main function of active sonar on submarines is searching for surface ships and other submarines, but avoidance of mines and sea mounts, and underwater communications, are also very important functions. Active search requires large projector arrays operating in the 2–10 kHz region for medium range performance, while obstacle avoidance uses smaller, higher-frequency arrays. All submarine applications require transducers capable of withstanding hundreds of pounds per square inch of hydrostatic pressure without significant change in performance.

The goal of both passive and active sonar systems is reliable long- range detection and ranging capability, but the basic considerations that influence performance of the two types of sonar are quite different. The receiving array in passive systems such as towed arrays or wide aperture ranging arrays must be able to detect signals with unknown frequency content, and therefore must operate over a frequency band much greater than the band of a typical active system. And they must do so in the presence of interfering noise. Chap. 4 shows that there are many ways to design hydrophones with adequate broadband sensitivity that are small, lightweight, and inexpensive compared to the high-power projectors needed for active sonar. But the main problem in passive sonar is control of the interfering noise, especially in ship-mounted arrays.

The previous six chapters presented the status of underwater sound transducers and arrays at the beginning of the twenty first century in considerable detail, but with a minimum of analytical background.

Most natural mechanisms and man-made devices are nonlinear, although linearity is often a good approximation and has been the basis for most engineering developments. In many devices the effects of nonlinearity become apparent only under high drive conditions, while other devices are inherently nonlinear and exhibit nonlinear effects, such as frequency doubling, for the smallest of drives.

Most natural mechanisms and man-made devices are nonlinear, although linearity is often a good approximation and has been the basis for most engineering developments. In many devices the effects of nonlinearity become apparent only under high drive conditions, while other devices are inherently nonlinear and exhibit nonlinear effects, such as frequency doubling, for the smallest of drives.

In this chapter we concern ourselves with the calculation of acoustic characteristics of transducers, such as directivity function, directivity factor, directivity index, and self radiation impedance. Convenient formulae and numerical information for frequently used cases will also be given.